POP (gelatin chloride printing out paper) is a century old formula with printing characteristics that cannot be found in any other contemporary printing paper. It has an exceptionally long tonal range, and its image colour can range from a ruddy brown through a rich chestnut to a violet-black according to the processing given each sheet.

The negative, which must be the same size as the print, is placed on the paper and exposed to high ultraviolet light (originally, direct sunlight). When the negative is removed, the image is already apparent prior to any immersion in chemicals. The print is then toned in a gold chloride solution, fixed, and washed. During each step of the processing, the tone of the print undergoes varying degrees of change. Thus no two finished prints will have exactly the same tone.